1. Field of the Invention
The present invention relates to a method of fabricating a plastic mask for paste printing for forming patterns by use of an ink, an adhesive agent, a solder paste, or a paste-like resin (hereinafter referred to as the paste) on a printing material, more particularly to a method of fabricating a plastic mask for such paste printing, which is carried out by forming a pattern of through-holes in a plastic sheet by excimer laser abrasion.
The present invention also relates to a plastic mask fabricated by the above-mentioned method, and also to a paste printing method by use of the plastic mask.
2. Discussion of Background
For forming a printed pattern on a printing material with the above-mentioned paste by use of a printing mask, the following method is generally used:
A mask with a pattern of through-holes is brought into close contact with a printing material, and a paste is spread on the surface of the printing mask by a squeegee so as to fill the through-holes with the paste. An excessive paste is then removed from the surface of the printing mask.
The printing mask is then separated from the printing material in such a manner that the paste passes through the through-holes and remains in the same pattern as that of the through-holes formed in the printing mask on the printing material, whereby a paste pattern corresponding to the pattern of the through-holes is printed on the surface of the printing material.
Such printing masks for use in practice can be roughly classified into screen masks and metal masks.
As screen masks, there are known, for example, punch press mask, YAG laser mask, etching mask, additive mask and half-etching mask.
The punch press mask was developed as a solder paste printing mask for fabricating electronic parts. The punch press mask can be prepared by forming round through-holes in a metal sheet with a punch.
This printing mask can be easily and quickly prepared by utilizing NC data of a mounter used in a mounting process with high through-hole making speed.
This printing mask has the advantages over other printing masks that the production method is simple and therefore the printing mask can be produced at extremely low cost. Furthermore, the apparatus for producing the printing mask is also inexpensive and small-sized and therefore can be brought into and used in a printing facility without difficulty.
Despite such advantages, this printing mask has the shortcomings that upper edge portions of the through-holes tend to be rounded, and rough broken surfaces or burrs tend to be formed in the lower edge portions of the through-holes, so that the paste filled in the through-holes cannot smoothly pass through the through-holes and therefore the printing quality is poor.
A minimum diameter of the through-hole that can be formed is about 0.3 mm. For example, in the solder paste printing for fabricating electronic parts, a minimum pitch will be about 0.5 mm in QFP (quad flat package) at best.
A YAG laser mask is prepared by fusing a metal sheet (usually SUS) with YAG laser and fused portions in the metal sheet are blown off by use of an assist gas to form through-holes in the metal sheet.
In the case of the YAG laser mask, through-holes are made in accordance with the NC data or the like, so that the YAG laser mask has the advantage that the mask can be produced in a short time with a minimum dispersion in quality, but has the disadvantages that the inner walls of the through-holes are rough, and dross is deposited at the edge portions of the through-holes, so that the printing quality obtained by the YAG laser mask is slightly inferior to that obtained by other printing masks.
Furthermore, convex portions are formed in the edge portions by the deposition of the dross thereon, and the thus formed convex portions tend to make imperfect the contact of the printing mask with a printing material during the printing process. If such imperfect contact takes place, it is necessary to remove the convex portions from the edge portions by grinding or polishing the surface of the printing mask.
An etching mask is prepared as follows: A photosensitive resin is coated on both sides of a metal plate to form resist layers thereon. The resist layers are exposed to light through a photomask on which through-holes are depicted in a pattern (hereinafter referred to as the through-hole pattern depicted photomask) and are then developed by removing the portions corresponding to the through-hole pattern from the resist layers. This metal plate is then subjected to etching, whereby through-holes are made in the same pattern as the above-mentioned through-hole pattern in the metal plate. Thus, an etching mask for printing is prepared. In the thus prepared etching mask, etching is carried out on both sides of the metal plate for making the through-holes therein, so that a convex portion is formed at the central portion of the inner wall of each through-hole. Furthermore, the inner wall of the through-hole is rough, so that a paste cannot smoothly pass through each through-hole. The result is that the printing quality obtained by the thus prepared etching mask is poor.
An additive mask is prepared as follows: A relatively thick photosensitive resin layer or dry film layer is provided on a plating substrate. The photosensitive resin layer is exposed to light through a through-hole pattern depicted photomask and developed by removing the portions in the photosensitive resin layer except the portions corresponding to the through-hole pattern. Thus, the photosensitive resin layer portions remain in the through-hole pattern on the plate substrate. A metal plate, for example, made of nickel, with an appropriate thickness, is formed by plating around the photosensitive resin layer portions which remain in the through-hole pattern. The metal plate is then removed from the plating substrate, and the photosensitive resin layer portions are dissolved away from the metal plate, whereby a metal plate with through-holes which correspond to the removed photosensitive resin layer portions is prepared as a printing mask.
The inner walls of the through-holes formed in the above metal plate are smoother than any of the inner walls of through-holes formed in metal masks prepared by any other methods and accordingly, the printing quality obtained by the metal plate is relatively good. However, occasionally, the inner walls of the through-holes are roughened by the over-exposure of the dry film, which may be referred to as "attack phenomenon".
It is considered that the "attach phenomenon" is caused as follows. It is necessary that the photosensitive resin layer or dry film for the formation of the through-holes in the printing mask be sufficiently thicker than the additive mask. Occasionally, the thickness of the photosensitive resin layer may be 200 .mu.m or more. Since this thickness is much greater than that of a photosensitive resin layer for the masking of an etching liquid, which is in the range of 20 to 30 .mu.m, a sufficient amount of light must be applied so as to have the light reach the bottom portion of the photosensitive resin layer on the side of the substrate. For this reason, light with high intensity is applied to the uppermost surface of the photosensitive resin layer on the side of the photomask.
When the above-mentioned light applied to the uppermost side of the photosensitive resin layer causes halation within the photosensitive resin layer, the portions which should be shaded by the photomask are exposed to the light scattered from the adjacent portions within the photosensitive resin layer. As a result, the inner walls of the through-holes are roughened as mentioned above.
Furthermore, stepped portions are formed in the inner walls of the through-holes with the attachment of the dry film thereto while in use, whereby the printing quality is lowered.
The plated surface of the printing metal plate which comes into contact with a printing material tends to be roughened. In particular, when finely-divided particles are contained in a paste for printing, such as in a solder paste, and such a paste spreads, the finely-divided particles in the paste enter the roughened portions at the plated surface of the mask and are fixed thereon, so that the roughness of the surface is further intensified. The result is that the close contact with the printing metal plate with the printing material is hindered, and the spreading of the paste is further increased.
On the other hand, one of the most representative material used as plating materials in the additive mask is nickel. Nickel is softer than stainless steel (usually SUS 304) which is frequently used as a material for an etching mask, so that the portions near minute through-holes are easily deformed by the convex portions in the roughened surface or by some foreign materials, whereby the printing quality tends to be easily degraded.
As a countermeasure to avoid this problem, the hardness of the additive mask is increased, for example, with the addition of cobalt thereto at the time of plating. This countermeasure, however, tends to increase the roughness of the plated surface and to promote the above-mentioned fixing of the finely-divided particles in the paste, thereby increasing the spreading of the paste.
A half-etching mask is used for printing, particularly a solder paste in printed circuit boards for electronic parts. In this field, it is required that a mixture of large and micro electronic parts be mounted in accordance with the recent tendency that electronic parts are small-sized. When solder paste printing is performed for micro electronic parts, it is required that the printing mask be made thin and that micro through-holes be formed in the printing mask and filled with the solder paste; while when solder paste printing is performed for relatively large parts, it is required that the printing mask be made thick and that relatively large through-holes be formed in the printing mask and filled with the solder paste.
When solder paste printing is performed on printed circuit board on which large parts and micro parts are to be mixedly mounted, by use of a thin printing mask for solder printing for micro parts, the solder paste for the large parts quickly runs out; while when solder paste printing is performed on the printed circuit board by use of a thick printing mask for solder printing for large parts, the solder paste does not smoothly pass through the micro through-holes for the micro parts, and even if the solder paste passes through the micro through-holes, excellent paste printing cannot be performed due to the presence of excessive solder paste.
It is the half-etching mask that can solve the above-mentioned problems since it is partially made thin by etching and paste printing for large parts and that for micro parts can be performed by use of one and the same printing mask.
The half-etching mask is prepared as follows: First of all, in accordance with the previously mentioned method for preparing the additive mask, there is prepared a printing mask with a necessary thickness for large parts, micro through-holes for micro parts, and relatively large through-holes for large parts.
The micro through-holes within a half-etching area are filled with a resin for protecting inner walls of the micro through-holes.
The area except the half etching area, for which etching is unnecessary, is masked by use of a resist agent, and then etching is performed until the thickness of the etched area reaches a suitable thickness for the micro parts, whereby a thin printing mask is prepared only in the area where micro through-holes for micro parts are formed.
Such a half-etching mask has made it possible to perform solder paste printing for large, normal, and micro electronic parts by use of one and the same printing mask in the field of electronic part mounting. On the other hand, even if the micro through-holes are filled with the resin for protecting the inner walls thereof, the spreading of the etching liquid cannot be completely prevented, so that the smooth inner walls made by the additive method are roughened by the spread etching liquid. The result is that the half-etching mask tends to have the shortcoming that the micro through-holes for most precise printing are most roughened and accordingly the printing quality is lowered.
Furthermore, the above-mentioned process for preparing the half-etching mask includes the step of carrying out the additive method, so that the previously mentioned shortcomings of the additive method may also appear in the above-mentioned process. Namely, the additive method has the shortcomings that the inner walls of the through-holes are roughened by the over-exposure of the dry film, or stepped portions are formed in the inner walls of the through-holes with the attachment of the dry film thereto while in use, whereby the printing quality is lowered.
Furthermore, the above-mentioned metal masks, such as punch press mask, YAG laser mask and etching mask, lack flexibility, so that when a printing material has convex portions on the surface thereof, even if it is tried to bring such a printing mask into close contact with the printing material, there are formed gaps between the printing mask and the printing material. Therefore, when the through-holes in the printing mask are filled with a paste, the paste enters the gaps between the printing mask and the printing material, so that the printed paste pattern improperly spreads. Furthermore, when a paste containing solid particles is employed, the solid particles adhere to the back side of the mask, and during the repeated printing by use of the mask, the adhering solid particles make a gap between the printing mask and the printing material, so that the improper spreading of the printed paste pattern is further increased.
Metal sheets (mainly a nickel sheet), such as the additive mask and the half-etching mask, which are prepared by plating, are flexible, so that when such a metal sheet is brought into close contact with a printing material, even if there are foreign materials or convex portions on the surface of the printing material, the mask can be easily deformed so as to be in close contact therewith. Therefore, when a portion near the through-holes of the mask is deformed and the mask is repeatedly used for printing, the paste enters behind the mask from the deformed portion thereof. The result is that the printed paste patterns spread and the printing quality is lowered.
Furthermore, the thicker the plated layer, the rougher the surface of the plated layer. In particular, when the paste contains solid particles which can be easily deformed, such as solder particles, such solid particles enter between the mask and the printing material and are depressed therebetween. The depressed solid particles adhere to the rough surface of the mask. When such a printing mask is repeatedly used for printing, the adhering solid particles form gaps between the printing mask and the printing material. When the through-holes of the mask are filled with a paste, the paste enters the gaps between the printing mask and the printing material, so that the printed paste pattern spreads.
When the adhering solid particles overflow out of the through-holes, the paste does not easily pass through the through-holes, so that the printing quality of the paste is degraded.
In the case where a paste enters the gap between a printing mask and a printing material, and the paste or solid particles contained in the paste adhere to the back side of the printing mask, it is required to wipe off the paste which adheres to the back side of the printing mask or to clean the back side of the printing mask quite often, especially when the printing mask is used repeatedly.
In accordance with the recent trend that electronic appliances and parts are made smaller and smaller in size, demands for highly precise paste printing of micro patterns with high quality free from non-printing portions or chips and spreading, are increasing in the field of the application of paste printing to electronic appliances and the like.
In order to solve the previously mentioned various problems of the conventional metal masks and to perform precise paste printing with high quality, thereby meeting the above-mentioned demands, the inventor of the present invention has prepared a plastic mask for paste printing by forming through-holes in a predetermined pattern in a plastic sheet by excimer laser abrasion, and has conducted paste printing by use of the plastic mask prepared by excimer laser abrasion. As a result, it has been discovered that the previously mentioned various problems of the conventional metal masks can be solved by the plastic mask.
Through-holes can be formed in a plastic sheet by excimer laser abrasion, so that the through-holes formed in the plastic sheet are free from burrs in the edges and deposition of dross as in the through-holes in the conventional metal masks, and also free from non-clear-cut edges as in the through-holes formed in the conventional punch press mask.
Furthermore, the inner walls of the through-holes formed in the plastic mask are extremely smooth and the formed through-holes themselves are precise and clean.
In a conventional metal mask made of stainless steel (usually SUS304), the surface of the metal mask is subjected to hair-line finishing or crape finishing. Further, in the case where the metal mask is made of an electro-forming nickel, the surface of the metal mask is rough. In such a case, it may occur that fine particles, for example, solder particles, contained in a solder paste which is spread onto the back side of the mask, are depressed between the mask and a printing material, so that the depressed particles are fixed to the rough surface of the metal mask. This is apt to occur at the convex portions in the printing surface of the mask where the paste tends to spread. When this takes place, the contact of the mask with the printing material becomes imperfect, so that the spreading of the paste is increased.
In contrast to this, in the case of the plastic mask, the surface thereof can be easily made extremely smooth, so that the fine particles contained in the paste are hardly fixed to the surface of the plastic mask. This significant feature of the plastic mask makes it possible to attain (a) close contact of the mask with a printing material, (b) significant reduction of the spreading of the paste, and (c) improvement of continuous printing performance.
Further, by use of the plastic mask, a so-called "half-etching mask" can be easily prepared, in which a portion of the plastic sheet on the side with which a squeegee comes into contact is partially made thin, and a predetermined pattern of through-holes for micro parts is formed in the partially thinned portion.
The plastic mask is softer and more flexible than the metal masks and therefore can flexibly follow any convex and concave portions on a printing surface to attain close contact therewith without any gap or with a minimum gap between the mask and the printing surface.
Furthermore, since the plastic mask is flexible, when it is peeled away from the printing surface, the plastic mask can immediately return to its original flat shape. In other words, in the plastic mask, there occurs no deformation as in a metal mask such as a nickel sheet formed by plating, when the mask is brought into close contact with a printing surface with convex and concave portions thereon.
Therefore, by performing paste printing by use of the plastic mask, the through-holes in the plastic mask can be completely filled with the paste, and when the plastic mask is removed from the printing material, the filled paste can pass through each through-hole in a complete shape, and the formation of gaps between the plastic mask and the printing surface can be minimized, so that on the printing surface, there can be formed a paste pattern which accurately corresponds to the pattern of the through-holes formed in the mask and is free from any spreading of the paste, whereby highly precise, high quality paste printing can be performed.
However, there are the following problems in the production of the plastic mask and also in the paste printing by use of the plastic mask:
(1) The plastic mask is usually prepared by placing a plastic sheet on a working table, then bringing the plastic sheet into close contact with the surface of the working table, and forming through-holes in the plastic sheet with the irradiation with an excimer laser beam from above the plastic sheet. In this method, however, in a portion of the plastic sheet with which the working table is not sufficiently in close contact, occasionally, there occurs that a plastic thin film remains at the bottom of through-holes made in the plastic sheet, which is hereinafter referred to as the "thin-film remaining phenomenon"; and in a portion of the plastic sheet with which the working table is completely in close contact, there occurs that the lower edges of the formed through-holes are rounded, which is hereinafter referred to as the "edge-rounding phenomenon".
When a through-hole is made in the plastic sheet by excimer laser, the through-hole is usually tapered with the opening of the through-hole becoming narrower in the direction from the upper opening to the lower opening thereof.
When paste printing is performed in practice by use of a plastic mask with such through-holes, the plastic mask is reversed upside down in such a manner that the narrower opening of the through-hole comes to the upper side and the larger opening of the through-hole comes to the lower side.
When the "thin-film remaining phenomenon" takes place, the presence of the remaining thin film at the upper end portion of the hole hinders the filling of the hole with the paste and the passing of the paste through the hole, thereby making ti impossible to perform paste printing with high quality and high precision.
Furthermore, when the "edge-rounding phenomenon" takes place, the rounded edge of the upper opening of the through-hole hinders the filling of the through-hole with the paste and the passing of the paste through the through-hole, thereby making it impossible to perform paste printing with high quality and high precision.
(2) The plastic mask is made of an insulating material and is softer than a metal, and therefore has problems due to the generation of electrostatic charges by the friction between the plastic mask and a squeegee during paste printing, and also due to abrasion resistance, which problems are not found in the metal masks:
As shown in FIGS. 32 and 33, when paste printing is performed, a paste 16 is spread on the surface of a plastic mask 50 by a squeegee 27, so that the through-holes 19, 20 and 21 arranged in a pattern in the plastic mask 50 are filled with the paste 16. As the squeegee 27, a soft and elastic squeegee made of, for example, urethane resin, is usually employed in order that the squeegee 27 can come into close contact with the surface of a thin half-etching portion 22 of the plastic mask 50, or in order that the squeegee 27 can come into close contact with the plastic mask 50 even if a printing material includes convex and concave portions at the surface thereof, or a curved surface. A squeegee made of urethane resin is an electrically insulating material.
Therefore, when friction is caused between the urethane squeegee and the surface of the plastic mask 50 during paste printing, the surface of the urethane squeegee and that of the plastic mask 50 are charged by the electrostatic charges generated at the respective surfaces thereof.
In the case of metal masks, since the metal masks themselves are electroconductive, there are almost no problems concerning the electrostatic charging. In contrast to this, in the case of the plastic mask, since the plastic mask itself is electrically insulating, problems caused by the electrostatic charging are unavoidable. In particular, serious problems such as electrostatic destruction of inner circuits are caused during actual operations for mounting electronic parts on printed circuit boards by duplex reflow soldering.
Duplex reflow soldering is performed as follows: A solder paste is printed on one side of a printed circuit board. Electronic parts are then mounted thereon by utilizing the viscosity of the solder paste. The electronic-parts-mounted printed circuit board is then passed through an electric furnace called "reflow furnace", whereby the solder paste is fused and the electronic parts are soldered on one side of the printed circuit board. This printed circuit board is turned upside down, and the above-mentioned soldering process is repeated on the opposite side of the printed circuit board, whereby electronic parts are soldered on both sides of the printed circuit board.
When the plastic mask is used in solder paste printing in the above-mentioned duplex reflow soldering, the following problems are caused:
Electrostatic charges generated by the use of the plastic mask are built up to reach a high potential in the plastic mask as triboelectric charging is repeated by the printing process. The thus built-up electrostatic charges flow into the electronic parts which are already mounted on one side of the print-circuit board, and when the electric charges reach a certain potential, there is the risk that micro circuits within the electronic parts are subjected to electrostatic destruction.
Some electronic parts which are soldered by the above-mentioned duplex reflow soldering are extremely vulnerable to electrostatic charges. For example, ROM, RAM, CMOS products and CCD are known as being apt to be subjected to electrostatic destruction by electrostatic charges of one hundred and several tens volts.
When a solder paste is printed on a glass epoxy substrate by use of a plastic mask without antistatic treatment and a urethane squeegee, the voltage of electrostatic charges built on the surface of the plastic mask actually exceeds as high as 1000 volts during 3 to 4 times printing operations. There is the risk that the previously mentioned electronic parts may be subjected to electrostatic destruction by the electrostatic charges with the above-mentioned high voltage.
Furthermore, with respect to the electronic parts which are electrostatically destroyed, the occurrence of the problems cannot be recognized from the external appearance thereof, and the problems are discovered during function inspection which is almost the final step of the mounting of the electronic parts, so that an enormous loss is incurred when the discovery of such problems is delayed.
The electrostatic destruction within the memory circuits such as ROM and RAM cannot be discovered by normal function inspection, so that there is the risk that inferior products may be put on the market.
As a countermeasure for avoiding the above-mentioned problems caused by electrostatic charging, the use of a squeegee made of a metal may be considered. To be more specifically, as the squeegee, a metal plate made of phosphor bronze may be used in an attempt to dissipate the electrostatic charges built on the surface of the plastic mask through such an electroconductive metal squeegee.
However, as shown in FIG. 34, a metal squeegee 67 is not as flexible as a urethane squeegee, so that when printing is performed at the half-etching portion 22 which is thinner than the other portion 51, the lower end portion of the metal squeegee 67 above the half-etching portion 22 passes over the thinner half-etching portion 22 without coming into close contact with the half-etching portion 22, since the other lower end portion of the metal squeegee 67 is supported on the other portion 51 which is thicker than the half-etching portion 22, so that a thin film of the solder paste 16 remains on the half-etching portion 22.
As a result, when the plastic mask 50 is peeled away from the printing surface, a thin film of the solder paste 16 remains on the half-etching portion 22 and hinders the passing of the solder paste 16 from the micro through-holes formed in the half-etching portion 22 as illustrated in FIG. 35. Therefore, the metal squeegee is not suitable for paste printing by use of a plastic mask including a half-etching portion, so that there is no choice but to use a plastic squeegee such as a urethane squeegee, and in such a case, the previously mentioned problems are caused by the triboelectric charging of the plastic squeegee.
Furthermore, since the plastic mask is softer than the metal masks, the plastic mask is inferior to the metal masks with respect to the abrasion resistance. In particular, when the printing material is a material having a rough surface, such as a ceramic plate and a glass epoxy layered substrate, or when hard and fine chips of the material for the substrate are present at the surface of the substrate, the abrasion of the plastic mask proceeds further quickly.
(3) A soft elastic squeegee, such as a squeegee made of urethane resin, is usually used in order to bring the squeegee into close contact with the thin half-etching portion 22, since the paste 16 is spread on the surface of the plastic mask 50 as illustrated in FIGS. 32 and 33.
Further as shown in FIG. 32, various sized through-holes, such as a large through-hole 19 and a medium through-hole 20 and a micro through-hole 21, are formed in the plastic mask 50 in such arrangement that corresponds to the arrangement of the connection terminals of electronic parts to be soldered. As illustrated in FIG. 36, when such through-holes are slit-shaped through-holes 41 which are formed side by side with a narrow rib 38 between each slit-shaped through-hole 41, and a urethane squeegee 27 passes over the slit-shaped through-holes 41 in contact therewith as illustrated in FIG. 37, the urethane squeegee 27 proceeds as the tip of the elastic urethane squeegee 27 slightly enters each of the slit-shaped through-holes 41 and catches each narrow rib 38, so that each rib 38 is bent as illustrated in FIG. 38. This is particularly apt to occur in small slit-shaped through-holes formed in a half-etching portion. This occurs more easily when the edge of the squeegee is directed so as to be in agreement with the longer side of the slit-shaped through-holes, in such a manner that the squeegee 27 is moved in the direction of the arrow as illustrated in FIG. 38.
When printing is performed as the ribs 38 are successively bent, the amount of the paste to be printed and the printing positions cannot be controlled properly. In the worst case, printing is carried out in such a manner that the adjacent slit-shaped through-holes are connected. Furthermore, if the bending of the ribs is repeated as the printing is performed, the base portions of each rib may be broken because of the fatigue thereof.